There have been known homopolypropylenes generally having excellent rigidity and heat resistance, and propylene block copolymers comprising both a polypropylene component and a rubber component and having excellent rigidity and heat resistance as well as excellent impact resistance.
Propylene polymers have also a low specific gravity and can be easily recycled, and therefore they have been paid much attention with respect to environmental protection and are now desired to be more extensively utilized.
Such propylene polymers are prepared using so-called a Ziegler-Natta catalyst comprising a compound containing a transition metal of Group IV to VI of the periodic table and an organometallic compound containing a metal of Group I to III, and they are widely used.
However, the propylene polymers obtained by the prior art techniques have not always sufficient rigidity and heat resistance in some uses, so that they have limited uses for some purposes.
It is known that the rigidity and the heat resistance of propylene polymers can be further improved by increasing the isotacticity of homopolypropylene or a polypropylene component in a propylene block copolymer, in other words, these properties can be improved by the use of a catalyst capable of providing a high isotacticity for the propylene polymers in the preparation thereof.
However, a polymer of an olefin such as propylene obtained by the use of such a catalyst capable of providing a high isotacticity tends to have molecular weights higher than those obtained by using conventional catalysts. Accordingly, it has generally been necessary to add hydrogen as a chain transfer agent in a large amount to the polymerization system in order to regulate a molecular weight and a melt flow rate (MFR) of the resulting polymer. Such a large amount of hydrogen present in the polymerization system, especially when propylene is per se used as the polymerization solvent, increases the pressure of the polymerization system, so that a polymerization reactor may need reinforcing of its pressure resistance.
Propylene block copolymers can be prepared by a multi-step polymerization (so-called block copolymerization) process which generally comprises initially polymerizing propylene to form a polypropylene component and then copolymerizing ethylene and an .alpha.-olefin to form a rubber component. If this polymerization process is carried out continuously (or in one batch) using the above-mentioned catalyst capable of providing a high isotacticity, a large amount of hydrogen gives rise to a problem. That is, the hydrogen added in the initial step to prepare the polypropylene component remains unreacted in a large amount and then, in the subsequent step, prevents the rubber component from attaining a high molecular weight (intrinsic viscosity (.eta.).
Accordingly, it has been desired that a catalyst system used for the preparation of a polypropylene and a propylene block copolymer be developed, which makes it possible not only to readily regulate the molecular weight and the melt flow rate (MFR) of the resulting polymers using a small amount of hydrogen, but also provide a high isotacticity for the resulting polypropylene and the propylene component of the resulting propylene block copolymer.
Further, it has also been desired that a process for preparing a propylene block copolymer by which the molecular weight and the melt flow rate (MFR) of the resulting copolymer can be easily regulated even with a small amount of hydrogen, isotacticity of a polypropylene component in the resulting copolymer can be heightened, and a molecular weight of a rubber component in the resulting copolymer can also be increased.